Dr. Universe, My fourth grade students and I want to know: What is wind? East Alton, IL

There’s wind in the Alton, Illinois, forecast this week, so it’s prime time for an answer. In fact, it looks like there’s wind all around Earth and even some gusts out on other planets.

My friend Nic Loyd, a meteorologist at Washington State University, studies all kinds of weather. He told me the simple answer: Wind is moving air.

We thought you might want to know more about how it works.

Wind happens both because of how the Sun heats up the Earth and the tiny air molecules that move around us all the time. You can’t see these air molecules, but they still have weight and take up space.

To experience this, just take a deep breath. As air fills up your lungs, they expand and your chest rises as you breathe in.

Oxygen and nitrogen make up most of Earth’s air molecules. As air heats up and cools down, it also exerts air pressure–which is like the weight of air–and a downward force. You may have experienced a change in air pressure if your ears popped in a car or airplane.

As the Sun heats up the Earth’s surface, differences in air pressure cause air to move. As it moves, it also balances out different air temperatures.

“A lack of wind would mean that some areas would get very hot, while other areas would become very cold,” Loyd explained.

As air warms up, it expands and its molecules spread apart. The air tends to weigh less and so it doesn’t exert too much air pressure. When air is cold, the molecules are packed tighter. The air weighs more and can exert more pressure.

Air usually flows from an area of high pressure to an area of low pressure. The colder air moves in to replace the warm air and we feel the wind.

We know that the warmest place on Earth is around the equator—the invisible belt around the middle of the planet, down by Ecuador, Kenya, and Indonesia. Warm air rises and flows out from the belt. It cools down and sinks as it moves toward the north and south poles.

If the Earth wasn’t spinning, the north winds would just go north and the south winds would go south.

“Of course, because the Earth does spin, things get a little more complicated,” Loyd said.

Wind wants to travel in that one direction, but the Coriolis effect makes wind move to the left or the right. Some winds in the northern hemisphere blow to the right, and winds in the southern hemisphere blow to the left.

As the wind moves, you can hear leaves rustling, watch it lift a kite up into the sky, see it expand a ship’s sails, or feel it cause hurricanes and storms. It’s pretty spectacular what tiny, invisible air molecules can do under pressure and the different temperatures.